1. Field of Invention
This invention relates to a method of increasing the strength and hardness of concrete, and decreasing its permeability to water. More specifically, it relates to methods of preparing a dense, hard surface on concrete, mortar, and grout which is stronger, harder, and more impervious to water than the original cemented material.
2. Prior Art
Current practice for making strong, impervious surfaces on concrete calls for the use of liners, coatings, or impregnation. Organic polymers such as latexes, epoxies, and acrylates are typically used to improve the strength and imperviousness of concrete. Polymers typically do not change the chemistry of the concrete but fill the pore spaces in the concrete and cover the surface to make the concrete less permeable to liquids. Polymer coatings do not increase the hardness and compressive strength of the concrete. Polymer impregnation can be used to improve the strength and hardness of concrete, but it involves the use of solvents, which cause air pollution.
Concrete may be treated in a vacuum chamber with silicon tetrafluoride gas, whereby calcium fluoride and silicon hydroxide are formed (L. H. Tuthill, "Resistance to Chemical Attack," ASTM Special Technical Publication No. 169, pp. 188-200, 1956.) This process has an adverse environmental impact and represents a health hazard because of the toxicity of silicon tetrafluoride. moreover, unwanted fluorides leach out of the concrete after treatment.
It is known that aqueous sodium metasilicate may be applied to the surface of the concrete to form a gel that fills the pores of the concrete. When the sodium leaches out, there may be a deleterious expansion when the alkaline solution reacts with opaline silicate aggregate in the concrete. Furthermore, because of the viscosity of aqueous sodium metasilicate solutions, penetration into the surface of the concrete is not deep.
Aqueous sodium silicate and silicofluoride solutions may be applied to the surface of concrete to form a low-permeability surface as the silicate reacts with the free lime in the concrete. This method is only effective with concrete whose surface is porous (Construction Repair, Palladin Publications, March/April 1994).
It is also known that the compressive strength of portland cement concrete may be increased by incorporating up to about 10% of reactive, amorphous silica in the concrete mixture which reacts with calcium hydroxide produced by the hydration of portland cement. The reaction of the calcium hydroxide and silica produces additional calcium silicate hydrate gel that bonds the aggregate particles in the concrete together.
U.S. Pat. No. 4,997,484 discloses a process in which fly ash, an alkali activator such as sodium hydroxide, and citric acid are incorporated to produce a cement that achieves high strength in a short curing time.
U.S. Pat. No. 4,306,912 discloses a process in which a short hardening time and early attainmnent of high strength are achieved by addition to the cement mixture of a sulfonated polyelectrolyte and sodium carbonate and/or sodium hydroxide.
U.S. Pat. No. 4,509,985 describes a process whereby early high strength is achieved by adding ground blast furnace slag to a mixture of aluminosilicate oxide, an alkali metal hydroxide, and an alkali metal polysilicate.
U.S. Pat. No. 3,047,407 discloses a method of waterproofing cement and concrete by preparing a mixture of a fatty soap, saponifiable resin soap, sodium hydroxide, and glycerin, stirring into this mixture a second mixture of aluminum chloride and potassium hydroxide, and combining this mixture with a third mixture of potassium hydroxide, caustic lime, and calcium chloride.
U.S. Pat. No. 4,234,347 discloses a binder for chemically resistant concrete comprising finely divided quartz having a specific surface area of 1000-5000 cm.sup.2 /gram and a cystalline modification of silica having alkali metal oxides on its surface. This binder is produced by mixing quartz sand with alkali metal carbonate and hydroxide and heating it to 1000.degree.-1550.degree. C.
U.S. Pat. Nos. 5,060,445 and 5,283,998 describe a roofing tile having an upper surface coated with an acrylic or asphaltic adhesive mixed with a fire-resistant material such as sodium carbonate or ammonium phosphate and light-weight concrete particles with decorative and heat-resistant particles covered by sodium silicate.